The cornea, which allows light transmission to the retina, is important for normal vision. Particularly, the corneal epithelium plays an essential role in preserving normal vision by maintaining the avascularity and transparency of the cornea, and thus, it is important that proper functioning of the corneal epithelium be maintained. Renewal and repair of the corneal epithelium are mediated by corneal epithelial stem cells located mainly in the limbus, the narrow region between the cornea and the bulbar conjunctiva. Damage or depletion of the corneal epithelial stem cells, known as limbal stem cell deficiency (LSCD), gives rise to conjunctival invasion, which causes vascularization of the cornea with an associated profound loss of vision. Occurrence of LSCD might result from external factors and disorders (for example, thermal or chemical injuries, microbial infections, surgeries involving the limbus, Stevens-Johnson syndrome, ocular cicatricial pemphigoid, aniridia, etc.).
Cell transplantation has been considered as a promising approach to reconstruct the corneal epithelium of patients with LSCD. For instance, researchers have attempted to use oral mucosal epithelial cells (OMECs), embryonic stem cells, conjunctival epithelial cells, epidermal stem cells, dental pulp stem cells, bone marrow-derived mesenchymal stem cells, hair follicle bulge-derived stem cells, umbilical cord lining stem cells, and orbital fat-derived stem cells for treating LSCD. Among the aforesaid therapeutic non-limbal cell types, conjunctival epithelial cells and OMECs are the only laboratory cultivated cell sources which have been explored in humans.
Both of the corneal and oral mucosal epithelia are stratified, with tight junction proteins (such as connexin 43 (Cx43)) in the suprabasal layer and hemidesmosome proteins (such as integrins) in the basal layer. Moreover, keratin 3/76 is expressed in both the corneal and oral mucosal epithelia. Due to the resemblance of the oral mucosal epithelia to the corneal epithelia, as well as the easy availability of the oral mucosal epithelium (i.e. no invasive surgery is required to harvest the oral mucosal epithelium), cultivated oral mucosal epithelial transplantation (COMET) has been widely used to repair damaged corneal surfaces and as an important bridge therapy for acute or chronic corneal burns. Recently, the COMET procedure has also been applied to repair intraoral mucosal defects and esophageal mucosa during endoscopic mucosal resection procedures, suggesting that such procedure has the potential for a wide variety of clinical applications.
Nakamura et al. and Nishida et al. reported the original protocol for the ex vivo cultivation of OMECs for COMET in 2004 (see Nakamura et al. (2004), Br. J. Ophthalmol. 88:1280-1284; and Nishida et al. (2004), N. Engl. J. Med. 351:1187-1196). Specifically, the original protocol uses dispase II/trypsin to isolate OMECs from tissues and to disrupt the epithelium. To cultivate the isolated OMECs ex vivo, fetal bovine serum (FBS) and 3T3 mouse fibroblasts (serving as feeder cells) are deemed necessary in the original protocol since they facilitate cell adhesion and proliferation which in turn lead to formation of a confluent epithelial cell sheet. Furthermore, researchers have verified the potency of COMET for promoting wound healing in severe ocular surface burns and demonstrated the long-term persistence of OMECs in the transplanted corneas.
However, FBS and mouse-derived 3T3 feeder cells are xenobiotic materials which might give rise to transmission of zoonotic infections or unknown pathogens. When it particularly comes to the ex vivo cell expansion for clinical application, the use of xenobiotic materials, such as animal-derived serums and feeders, might increase the risk of transmission of diseases (e.g. bovine spongiform encephalitis). Therefore, an animal-derived component-free (ADCF) culture procedure, in particular a serum-free and feeder-free culture procedure, is in demand for the next generation of COMET.